It is informed that, the present invention relates to an ultrasonic transducer applicable to Korean Patent Application No. 10-2004-0042299, “High directional ultrasonic ranging measurement system and method in air using parametric array”, commonly assigned to the applicant of the present invention and, U.S. patent application Ser. No. 10/960083 and Japanese Patent Application No. 2005-169938, “Ultrasonic ranging system and method thereof in air by using parametric array”, both based on a priority of the aforementioned Korean Patent Application.
Herein, the term “ultrasonic transducer” indicates both a unit of an ultrasonic actuator and an ultrasonic sensor, serving as a transmitter and a receiver, respectively, together in a pulse-echo ranging measurement method using ultrasonic waves.
An ultrasonic ranging measurement method using parametric transmitting array will be first described with reference to FIG. 1.
If an ultrasonic actuator serving as a high frequency generator radiates a high pressure ultrasonic wave (primary wave) having two frequency components, f1 and f2, toward a same direction, the ultrasonic wave radiated will form a high directional difference frequency wave (i.e., a secondary wave, fd=f1−f2) for a ranging measurement due to nonlinear effects while progressing. Ultimately, if the difference frequency wave comes back after being reflected from a target while progressing, an ultrasonic sensor will receive this reflected wave.
A combination of an ultrasonic actuator and an ultrasonic sensor serves as an ultrasonic transducer for generating and sensing pulse signals required in a ranging measurement.
There has been a variety of conventional ultrasonic transducers according to a driving method thereof such as a voice coil transducer, a capacitive transducer, a piezoelectric transducer, a magnetostrictive transducer, and a MUT (Micro-machined Ultrasonic Transducer) manufactured via MEMS.
Among the aforementioned transducers, the piezoelectric and the capacitive transducer have been the most generally used transducers for a ranging measurement at present. In case of the piezoelectric transducer, an actuator for transmitting and a sensor for receiving are generally detached, whereas the capacitive transducer is often used as a reversible transducer which serves both as a transmitter and a receiver.
It is noted that, the MUT type transducer using a piezoelectric body as a driving material is called a pMUT (piezoelectric Micro-machined Ultrasonic Transducer), and because of the characteristics of MEMS, the pMUT is suitable for an ultrasonic transducer with high frequency. Further, because of the small size of the unit ultrasonic actuators, the pMUT generally has a plurality of small unit ultrasonic actuators arranged therein for improving its output power.
The aforementioned ultrasonic transducers known so far are, however, not suitable for a ranging measurement using a parametric transmitting array. Hereinafter, the cause for such problem, a plan to overcome the problem and the like will be separately explained for the cases of the ultrasonic actuator and the sensor.
Ultrasonic Actuator
In order to generate a difference frequency wave by forming a parametric transmitting array, an ultrasonic actuator is essentially required to generate an ultrasonic wave having two frequency components toward a same direction (Condition I). Moreover, the ultrasonic actuator is needed to generate the difference frequency wave with high efficiency due to the weak nonlinear properties in air. In other words, in order to achieve a good efficiency of generating the difference frequency wave, first, the wave is required to be generated to have a large sound pressures (p0=p1=p2, Condition II); second, the transducer is needed to have a large size (Condition III); and lastly, the frequency difference between the two high frequencies (fd=f1−f2) is required to be large (Condition IV).
However, there are some practical limits in use of the transducer for a ranging measurement in air. In particular, in order to be applied to a small sized system such as a robot, the transducer needs to be very small, which does not satisfy the condition III. Further, there is a limit on increasing the frequency difference between the two frequency components because the attenuation effect of the ultrasonic waves is in proportion to the second power of the frequency, which fails the condition IV.
Accordingly, an ultrasonic actuator should be designed to maximize the condition II while satisfying the condition I. In other words, it is required to provide an ultrasonic actuator capable of having a large output at two frequency bands while maintaining a small size.
However, all of the conventional ultrasonic actuators correspond to a single resonance type, thereby failing to provide a sufficient output at two frequency bands. In addition, if an ultrasonic actuator is designed to be kept in small size, the radiation area thereof will also get small, and as a result, the output thereof will be too small. It is, therefore, impossible to apply such actuators to a ranging measurement using a parametric transmitting array.
Ultrasonic Sensor
In accordance with the conventional ultrasonic ranging measurements, the frequency of the wave generated by an ultrasonic actuator and that of the wave measured by an ultrasonic sensor are identical, thus the ultrasonic sensor has the same resonance frequency as that of the ultrasonic actuator.
However, in a ranging measurement method using a parametric transmitting array, the frequency band of the ultrasonic wave primarily generated by an actuator is very different from that of the ultrasonic wave measured by a sensor. It is, therefore, very difficult to apply such conventional sensors to a ranging measurement using a parametric transmitting array.